The present invention generally relates to a constant current semiconductor device, and in particular to a constant current semiconductor device which may be used as a current limiter such as a constant current diode.
As is well known, constant current limiters intended to prevent an excessive current from flowing in a circuit are widely used. Conventionally, a transistor is used as a constant current limiter. Recently, constant current limiters implemented with a semiconductor diode have been proposed. An example of these constant current limiters is disclosed in Zuleeg, IEEE, EDL-1 No. 11, pp. 234, 1980. The current limiter reported by Zuleeg is a constant current diode composed of a cathode electrode, an n.sup.+ -type GaAs cathode side contact layer, an n-type GaAs active layer, an n.sup.+ -type GaAs anode side contact layer and an anode electrode. A feature of this current limiter is that a thickness of the active layer is approximately 0.5 .mu.m and is very thin. Therefore, electrons may move in the active layer without being affected by lattice vibration and impurity scattering. In other words, ballistic transport of electrons can be realized. When electrons are in the ballistic transport state, the speed of electrons can reach the highest speed which is allowable in semiconductors. As a result, in the case where the density of electrons in the ballistic transport state is the same as that in the collision dominated transport state, the current density in the ballistic transport state becomes highest. In other words, the ballistic transport of electrons make it possible to fabricate a current limiter having the same limited current with the smallest semiconductor area. In this case, almost all of a voltage applied to the current limiter is converted into a kinetic energy .epsilon. of electrons. For this reason, when the applied voltage is almost the same as .DELTA.E.sub.GaAs /e (where .DELTA.E.sub.GaAs is an intervalley energy difference at the active layer and e is an absolute value of charge of an electron), the intervalley transfer of electrons can be realized in which the electrons transfer from an .GAMMA. valley into an L valley of the active layer, and thus a current resulting from motion of the electrons is saturated. Hence, the current becomes constant at the above voltage. The applied voltage causing the current saturation is called a current saturating voltage.
As described above, the current saturating voltage depends on the .DELTA.E.sub.GaAs, which is an inherent value of the GaAs active layer. Accordingly, a value of the current saturating voltage is fixed. In other words, it is impossible to select arbitrary values of the current saturating voltage. Generally, the current limiter is used for protection against excessive currents passing through circuits. Therefore, it is required that the current limiter should have an operational condition optimized for a circuit to which the current limiter is applied. This means that the conventional current limiter is useful for only limited applications.
Currently, active elements which are constituted by laminating compound semiconductor layers in one direction have been proposed and fabricated as electronic devices. Examples of these are a resonant-tunneling diode (generally called an RD), a resonant-tunneling hot electron transistor (generally called an RHET), a high electron mobility transistor (generally called an HEMT), a heterojunction bipolar transistor (generally called an HBT) and a resonant-tunneling bipolar transistor (generally called an RBT). As described above, these semiconductor devices are active elements constructed by growing compound semiconductor layers in one direction. In order to protect circuits formed by the RBT, RHET, HBT or HEMT against excessive currents, it is desired to build the current limiter having laminated compound semiconductor layers in one direction in the structure of the RBT, RHET, HBT or HEMT. In other words, the current limiter having a structure matching with the structure of the RBT, RHET, HBT or HEMT is desired.